The present invention relates to recording on magnetic media and more particularly to a system for varying the recording bias dynamically so that at each instant the bias is more nearly optimized for the spectrum of material to be recorded than when a fixed bias is employed. In the magnetic recording of analog signals, for example audio, it is normal to employ high frequency recording bias to linearize the otherwise highly nonlinear medium. The current applied to the recording head then consists of the addition of a current representing the analog signal to be recorded and a bias current waveform, normally sinusoidal, at a frequency several times higher than the highest frequency component of the analog signal. The magnitude of the bias current affects various parameters of the recording system: signal-to-noise ratio, harmonic distortion, frequency response, sensitivity, output level, modulation noise and the incidence of drop-out (momentary drops in output level due to inconsistencies in the magnetic tape and in its contact with the recording head). For many typical recording situations, particularly those where the recording medium is a Compact Cassette, the primary concern is a conflicting requirement between a magnitude of bias current that results in minimum distortion at low and middle audio frequencies along with minimum drop-outs and a lower bias current magnitude that provides a higher maximum output level at high audio frequencies. Heretofore, bias level has been chosen as a compromise among the various tape parameters that vary with the bias level. In the case of Compact Cassette recording, tape saturation occurs at high audio frequencies, thus limiting the high frequency maximum output level and causing an audible loss in the high frequency range of reproduced tapes. High frequency tape saturation in Compact Cassette recording can be substantially reduced, although apparently not completely eliminated by optimizing record bias for that result. Although saturation does not generally occur at high frequencies in reel-to-reel tape recorders operated at professional speeds, for example, 38 cm/sec (15 ips) or 19 cm ( 71/2 ips), the high frequency output level and other record parameters of such recorders are enhanced by bias current levels lower than compromise values generally used. However, the low and middle frequency distortion, drop-out rate and other parameters are degraded by such lower bias magnitude.
Since the conflicting requirement between a bias level chosen for optimum low and mid-frequency audio versus a bias level chosen for optimum high frequency audio output level is basic to typical recording situations and because the results of that selection are so readily audible in the reproduced recording, the emphasis of the application and the preferred embodiments are directed to the resolution of the conflict. Those of ordinary skill in the art will appreciate that circumstances, particularly the type of magnetic medium, effective recording speed or specially desired results may cause other record parameters to be of greater importance.
While the selection of a preferred bias level at any given frequency still requires a compromise, because the parameters tend to be optimum at different bias levels even for a given frequency, a substantial improvement in overall performance can nevertheless result compared to a compromise bias level for the entire frequency spectrum of interest to be recorded. The choice of bias level therefore requires information on the frequency spectrum to be recorded.
It should be noted that for each value of bias current the recording amplifier gain and equalization can be set to give the desired level of recorded magnetic flux and the desired overall (record-playback) frequency response, provided of course that the input level is not so high as to result in tape non-linearity.